[flang] Improvements to semantics tests

Change test scripts so that cmake passes in the path to the `f18`
that it just built. They no longer have to use relative paths.

Simplify argument passing in test scripts by sharing it in `common.sh`.

Move `if_arith01.f90`, `if_construct01.f90`, `if_stmt01.f90` to `ERROR_TESTS`.
As it was, nothing was being tested on them.

For `test_error.sh`, find the include directory relative to `f18` and
pass that in with a `-I` option. Eliminate the temporary definitions of
`iso_fortran_env`, `iso_c_binding`, `ieee_exceptions` in the tests
because the "real" ones are now available.

The tests that are run with `test_any.sh` could probably have similar
changes, but they are not part of this commit.

Original-commit: flang-compiler/f18@b62fe489be9aefeebb11954a6436285a1b5e6840
Reviewed-on: https://github.com/flang-compiler/f18/pull/568